Displacement front behavior of near miscible CO2 flooding in decane saturated synthetic sandstone cores revealed by magnetic resonance imaging

It is of great importance to study the CO₂-oil two-phase flow characteristic and displacement front behavior in porous media, for understanding the mechanisms of CO₂ enhanced oil recovery. In this work, we carried out near miscible CO₂ flooding experiments in decane saturated synthetic sandstone cores to investigate the displacement front characteristic by using magnetic resonance imaging technique. Experiments were done in three consolidated sandstone cores with the permeabilities ranging from 80 to 450 mD. The oil saturation maps and the overall oil saturation during CO₂ injections were obtained from the intensity of magnetic resonance imaging. Finally the parameters of the piston-like displacement fronts, including the front velocity and the front geometry factor (the length to width ratio) were analyzed. Experimental results showed that the near miscible vertical upward displacement is instable above the minimum miscible pressure in the synthetic sandstone cores. However, low permeability can restrain the instability to some extent.     

Study of the fluid flow characteristics in a porous medium for CO2 geological storage using MRI

The objective of this study was to understand fluid flow in porous media. Understanding of fluid flow process in porous media is important for the geological storage of CO₂. The high-resolution magnetic resonance imaging (MRI) technique was used to measure fluid flow in a porous medium (glass beads BZ-02). First, the permeability was obtained from velocity images. Next, CO₂–water immiscible displacement experiments using different flow rates were investigated. Three stages were obtained from the MR intensity plot. With increasing CO₂ flow rate, a relatively uniform CO₂ distribution and a uniform CO₂ front were observed. Subsequently, the final water saturation decreased. Using core analysis methods, the CO₂ velocities were obtained during the CO₂–water immiscible displacement process, which were applied to evaluate the capillary dispersion rate, viscous dominated fractional flow, and gravity flow function. The capillary dispersion rate dominated the effects of capillary, which was largest at water saturations of 0.5 and 0.6. The viscous-dominant fractional flow function varied with the saturation of water. The gravity fractional flow reached peak values at the saturation of 0.6. The gravity forces played a positive role in the downward displacements because they thus tended to stabilize the displacement process, thereby producing increased breakthrough times and correspondingly high recoveries. Finally, the relative permeability was also reconstructed. The study provides useful data regarding the transport processes in the geological storage of CO₂.     

Optimization of gas pressures ratio in a fast-axial-flow CO2 laser with genetic algorithm

In this paper, the laser output power of a fast-axial flow CO₂ laser was optimized with gas pressures ratio of CO₂:N₂:He using a genetic algorithm technique. The power of laser was increased from 500 W (un-optimized case) to 2200 W (simulated case), also experimentally the power has achieved the value of 700 W (optimized case).     

High-resolution transmission measurements of CO2 at high temperatures for industrial applications

High-resolution transmission spectra of CO₂ in the 2.7, 4.3 and 15 μm regions at temperatures up to 1773 K and at approximately atmospheric pressure (1.00±0.01 atm) are measured and compared with line-by-line calculations based on the HITEMP-1995, HITEMP-2010, CDSD-HITEMP and CDSD-4000 databases. The spectra have been recorded in a high-temperature flow gas cell and using a Fourier transform infrared (FTIR) spectrometer at a nominal resolution of 0.125 cm^?1. The volume fractions of CO₂ in the measurements were 1, 10 and 100%. The measurements have been validated by comparison with medium-resolution data obtained by Bharadwaj and Modest [6]. The deviations between the experimental spectra and the calculations at 1773 K and the vibrational energy exchange and thermal dissociation of CO₂ at high temperatures are discussed.     

Magnetic resonance imaging on CO(2) miscible and immiscible displacement in oil-saturated glass beads pack

In this study, the displacement processes were observed as gaseous or supercritical CO₂ was injected into n-decane-saturated glass beads packs using a 400-MHz magnetic resonance imaging (MRI) system. Two-dimensional images of oil distribution in the vertical median section were obtained using a spin-echo pulse sequence. Gas channeling and viscous fingering appeared obviously in immiscible gaseous CO₂ displacement. A piston-like displacement front was detected in miscible supercritical CO₂ displacement that provided high sweep efficiency. MRI images were processed with image intensity analysis methods to obtain the saturation profiles. Final oil residual saturations and displacement coefficients were also estimated using this imaging intensity analysis. It was proved that miscible displacement can enhance the efficiency of CO₂ displacement notably. Finally, a special coreflood analysis method was applied to estimate the effects of capillary, viscosity and buoyancy based on the obtained saturation data.     

PM10 formation during the combustion of N2-char and CO2-char of Chinese coals

The formation of PM₁₀ (particles less than or equal to 10 μm in aerodynamic diameter) during char combustion in both air-firing and oxy-firing was investigated. Three Chinese coals of different ranks (i.e., DT bituminous coal, CF lignite, and YQ anthracite) were devolatilized at 1300 °C in N₂ and CO₂ atmosphere, respectively, in a drop tube furnace (DTF). The resulting N₂-chars and CO₂-chars were burned at 1300 °C in both air-firing (O₂/N₂ = 21/79) and oxy-firing (O₂/CO₂ = 21/79). The effects of char properties and combustion conditions on PM₁₀ formation during char combustion were studied. It was found that the formation modes and particle size distribution of PM₁₀ from char combustion whether in air-firing or in oxy-firing were similar to those from pulverized coal combustion. The significant amounts of PM_0.5 (particles less than or equal to 0.5 μm in aerodynamic diameter) generated from combustion of various chars suggested that the mineral matter left in the chars after coal devolatilization still had great contributions to the formation of ultrafine particles even during the char combustion stage. The concentration of PM₁₀ from char combustion in oxy-firing was generally less than that in air-firing. The properties of the CO₂-chars were different from those of the N₂-chars, which was likely due to gasification reactions coal particles experienced during devolatilization in CO₂ atmosphere. Regardless of the combustion modes, PM₁₀ formation in combustion of N₂-char and CO₂-char from the same coal was found to be significantly dependent on char properties. The difference in the PM₁₀ formation behavior between the N₂-char and CO₂-char was coal-type dependent.     

Continuous measurements of stable carbon isotopes in CO2 with a near-IR laser absorption spectrometer

A near-IR laser absorption spectrometer using a technique of wavelength modulation spectroscopy is used to measure stable carbon isotope ratios of ambient CO₂ (δ¹³C) via the absorption lines ¹²CO₂ R(17) (2ν₁ + ν¹₂ − ν¹₂ + ν₃) at 4978.205 cm⁻¹ and ¹³CO₂ P(16) (ν₁ + 2ν₂ + ν₃) at 4978.023 cm⁻¹. The isotope ratios are measured with a reproducibility of 0.02‰ (1σ) in a 130-s integration time over a 12-h period. The humidity effect on δ¹³C values has been evaluated in laboratory experiments. The δ¹³C values of CO₂ in ambient air were measured continuously over 8 days and agreed well with those from isotope ratio mass spectrometry of canister samples. The spectrometer is thus capable of real-time, in situ measurements of stable carbon isotope ratios of CO₂ under ambient conditions.     

Investigation of the doped transition metal promotion effect on CO2 chemisorption on Ni (1 1 1)

The chemisorption of CO₂ on the pure Ni (1 1 1) and doped Ni (1 1 1) by transition metal (Co, Rh, Cr, Ce, La) were investigated by using the generalized gradient approximation (GGA) and the Perdew–Burke–Emzerhof (PBE) functional. The optimized structure of doped metal surface showed that Rh, Cr, Ce, La atoms upward shift from the surface of Ni (1 1 1) plane, while the atom radius of Co is the minimum offset which lead to the height is ?0.03 Å. The ability of CO₂ chemisorption follows the order of La/Ni (1 1 1) > Ce/Ni (1 1 1) > Cr/Ni (1 1 1) > Co/Ni (1 1 1) > pure Ni (1 1 1). It is exothermic when CO₂ chemisorbed on Cr/Ni (1 1 1) Ce/Ni (1 1 1) and La/Ni (1 1 1), while it is endothermic on the Co/Ni (1 1 1) and pure Ni (1 1 1). CO₂ molecular chemisorbed on all the metal surfaces are negatively charged, result from the electron transfer between the metal surfaces and the CO₂ molecular. The transition metals La, Ce and Cr can promote the transformation of electron and make the CO bonds longer than the pure Ni (1 1 1). We also analyzed the dissociation of CO₂ on the Ni-based surface and found that the La/Ni (1 1 1) surface is the preference surface for the dissociation of CO₂, which improved the ability to hinder carbon deposition.     

Experimental proof on two-cone axisymmetric-folded combination CO2 laser

The experimental proof of the light output on the two-cone axisymmetric-folded combination (ASFC) CO₂ laser has been performed. The output power from the centre discharge tube is 26.7 W, and that of one couple of folded discharge tubes is 40.5 W. Seventeen beams can be obtained from the device, which are from the folded cavities with axes placed in the inner and outer cones, respectively. Therefore, the ASFC CO₂ laser with more discharge tubes can be fabricated and much higher output power can be obtained.     

CO and H2O adsorption and reaction on Au(310)

We have studied desorption of ¹³CO and H₂O and desorption and reaction of coadsorbed, ¹³CO and H₂O on Au(310). From the clean surface, CO desorbs mainly in, two peaks centered near 140 and 200 K. A complete analysis of desorption spectra, yields average binding energies of 21 ± 2 and 37 ± 4 kJ/mol, respectively. Additional desorption states are observed near 95 K and 110 K. Post-adsorption of H₂O displaces part of CO pre-adsorbed at step sites, but does not lead to CO oxidation or significant shifts in binding energies. However, in combination with electron irradiation, ¹³CO₂ is formed during H₂O desorption. Results suggest that electron-induced decomposition products of H₂O are sheltered by hydration from direct reaction with CO.     

Effect of high CO2 concentration on char formation through mineral reaction during biomass pyrolysis

O₂/CO₂ combustion has attracted considerable attention as a promising technology for CO₂ capture. Using biomass for fuel is considered carbon neutral, and O₂/CO₂ biomass combustion can mitigate the deleterious environmental effect of greenhouse. In this study, the effect of CO₂, the main component gas in O₂/CO₂ combustion, on the pyrolysis characteristics of biomass is investigated. Cellulose, lignin, and metal-depleted lignin pyrolysis experiments were performed using a thermobalance. Information on the surface chemistry of the chars was obtained by Fourier transform infrared (FTIR) spectroscopy to investigate changes in the surface chemistry during pyrolysis under different surrounding gasses. When the temperature increased to 1073 K at heating rate of 1 K s^?1, the char yield of lignin in the presence of CO₂ increased by about 10% compared with that under Ar. However, for cellulose and metal-depleted lignin, no significant difference appeared between pyrolysis under CO₂ and that under Ar. FT-IR showed that a strong peak corresponding to carbonate ions appeared in the char derived from lignin under CO₂. Therefore, salts such as Na₂CO₃ or K₂CO₃ formed during the lignin pyrolysis under CO₂. At around 1650–1770 cm^?1, a significant difference appeared in the FTIR spectra of chars formed under CO₂ and those formed under Ar. C=O groups not associated with an aromatic ring were found only in chars formed under CO₂. It was suggested that these salts affected the char formation reaction, in that the char formed during lignin pyrolysis under CO₂ had unique chemical bands that did not appear in the lignin-derived char prepared under Ar.     

Measurements of H2O2 in low temperature dimethyl ether oxidation

H₂O₂ is one of the most important species in dimethyl ether (DME) oxidation, acting not only as a marker for low temperature kinetic activity but also responsible for the “hot ignition” transition. This study reports, for the first time, direct measurements of H₂O₂ and CH₃OCHO, among other intermediate species concentrations in helium-diluted DME oxidation in an atmospheric pressure flow reactor from 490 to 750 K, using molecular beam electron-ionization mass spectrometry (MBMS). H₂O₂ measurements were directly calibrated, while a number of other species were quantified by both MBMS and micro gas chromatography to achieve cross-validation of the measurements. Experimental results were compared to two different DME kinetic models with an updated rate coefficient for the H + DME reaction, under both zero-dimensional and two-dimensional physical model assumptions. The results confirm that low and intermediate temperature DME oxidation produces significant amounts of H₂O₂. Peroxide, as well as O₂, DME, CO_, and CH₃OCHO profiles are reasonably well predicted, though profile predictions for H₂/CO₂ and CH₂O are poor above and below ～625 K, respectively. The effect of the collisional efficiencies for the H + O₂ + M = HO₂ + M reaction on DME oxidation was investigated by replacing 20% He with 20% CO₂. Observed changes in measured H₂O₂ concentrations agree well with model predictions. The new experimental characterizations of important intermediate species including H₂O₂, CH₂O and CH₃OCHO, and a path flux analysis of the oxidation pathways of DME support that kinetic parameters for decomposition reactions of HOCH₂OCO and HCOOH directly to CO₂ may be responsible for model under-prediction of CO₂. The H abstraction reactions for DME and/or CH₂O and the unimolecular decomposition of HOCH₂O merit further scrutiny towards improving the prediction of H₂ formation.     

Effect of Li2CO3 addition on the structural,optical, ferroelectric,and electric-field-induced strain of lead-free BNKT-based ceramics

In this work, we reported the effect of Li₂CO₃ addition on the structural, optical, ferroelectric properties and electric-field-induced strain of Bi_0.5(Na,K)_0.5TiO₃ (BNKT) solid solution with CaZrO₃ ceramics. Both rhombohedral and tetragonal structures were distorted after adding Lithium (Li). The band gap values decreased from 2.91 to 2.69 eV for 5 mol% Li-addition. The maximum polarization and remanent polarization decreased from 49.66 μC/cm² to 27.11 μC/cm² and from 22.93 μC/cm² to 5.35 μC/cm² for un-doped and 5 mol% Li- addition BNKT ceramics, respectively. The maximum S_max/E_max value was 567 pm/V at 2 mol% Li₂CO₃ access. We expected this work will help to understand the role of A-site dopant in lead-free ferroelectric BNKT materials.     

Stability information in plasma image of high-power CO2 laser welding

In deep penetration laser welding, a capillary called keyhole is formed when the energy intensity reaches 10⁶ W/cm². During this process, the vaporized metal and the surrounding atmosphere can be ionized to form plasma both in and above the keyhole. The stability of the keyhole has an important influence on the properties of welded components and the fluctuations of plasma. In this paper, a method was developed to acquire the stability information from plasma images taken by high-speed photography. The influences of surface impurity and the flowrate of side-assist gas on the stability were investigated. Bead-on-plate welding was performed on 12 mm E-grade shipbuilding steel plates using a 15 kW CO₂ laser, with helium as the blowing gas. Three characteristic parameters were used to evaluate the stability. It was found that these three characteristic parameters can effectively indicate the stability variation caused by the surface impurity and gas flowrate. The present research provides important insights into developing image-based sensors to monitor the welding process.     

On the stability of the CO adsorption-induced and self-organized CuPt surface alloy

The stability of the recently discovered CO-induced and self-organized CuPt surface alloy was explored at near ambient pressures of O₂ (200 mbar) at room temperature, in a CO + H₂ mix (P_tot = 220 mbar, 4% CO) from room temperature to 573 K, as well as in a CO + H₂O mix (P_tot = 17 mbar, 50% CO) from room temperature to 673 K. No indications of substantial changes in surface structure were observed under the latter conditions compared to CO alone whereas the O₂ oxidation resulted in CO removal and the build-up of an ultrathin CuO_x-film. However, the oxidized CO/CuPt surface alloy could be regenerated by reducing the CuO_x in 100 mbar CO for 10 min at room temperature. The results show, amongst others, the stability of the CuPt surface alloy in various environments containing CO and how a novel coinage/Pt-group bimetallic surface alloy catalyst induced by CO adsorption can be reactivated before use in applications such as electrochemistry at ambient temperatures.     

A mechanistic char oxidation model consistent with observed CO2/CO production ratios

Reliable prediction of char conversion, heat release, and particle temperature during heterogeneous char oxidation relies upon quantitative calculation of the CO₂/CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of char combustion have been calibrated against the temperature-dependence of the CO₂/CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO₂/CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO₂/CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions characteristic of pressurized, oxy-fuel, and conventional high-temperature char combustion, for which essentially no experimental information on the CO₂/CO production ratio is available. The results suggest that the CO₂/CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO₂/CO production ratio results, an analytical procedure for calculating the CO₂/CO production ratio is presented.     

A simple dye-sensitized solar cell sealing technique using a CO2 laser beam excited by 60 Hz AC discharges

Dye-sensitized solar cells (DSSCs) use two glass substrates (photo electrode and counter electrode) coated with fluorine-doped tin oxide (FTO) to harvest light into the cell and to collect electrons. The space between the photo electrode and the counter electrode are filled with a liquid type electrolyte for electron transfer into the cell. Therefore, an appropriate sealing method is required to prevent the liquid electrolyte leaking out. In this paper, a simple CO₂ laser beam with TEM₀₀ mode excited by a 60 Hz AC discharge was used to seal two glass substrates coated with FTO for the fabrication of DSSCs. The sealing technique improved the durability and stability of the DSSCs. The optimal conditions for the sealing of the DSSCs are related to the pin-hole diameter, the discharge current and the moving velocity of the target. Especially, the CO₂ laser beam is used as a heat source that is precisely controlled by the pin-hole, which plays an important role in adjusting its spot size. From these results, the maximum laser power was found to be 40 W at 18 Torr and 35 mA. In order to achieve the best sealing quality, the following parameters are required: a pin-hole diameter of 4 mm, input voltage of 10.73 kV, discharge current of 9.31 mA, moving velocity of 1 mm/s and distance from the target surface of 26.5 cm. Scanning electron microscope images show that the sealing quality obtained using the CO₂ laser beam is superior to that obtained using a hot press or soldering iron.     

In-situ formation of SiC nanocrystals by high temperature annealing of SiO2/Si under CO: A photoemission study

We have studied CO interaction with SiO₂/Si system at high temperature (~ 1100 °C) and 350 mbar by core-level photoemission. Even for short annealing time (5 min) the signal from Si2p and C1s core levels shows a clear change upon CO treatment. Shifted components are attributed to formation of SiC. This is confirmed by TEM imaging which further shows that the silicon carbide is in the form of nano-crystals of the 3C polytype. Photoemission spectroscopy moreover reveals the formation of silicon oxicarbide which could not be evidenced by other methods. Combining these results with previous Nuclear Resonance Profiling study gives a deeper insight into the mechanisms involved in the nanocrystals growth and especially for the reaction equation leading to SiC formation. We show that CO diffuses as a molecule through the silica layer and reacts with the silicon substrate according the following reaction: 4 CO + 4 Si → SiO₂ + 2SiC + SiO₂C₂.     

Synthesis of Hierarchical (BiO)2CO3 Nanosheets Microspheres toward Efficient Photocatalystic Reduction of CO2 into CO

In this paper, hierarchical (BiO)₂CO₃ nanosheets microspheres were synthesized with dry ice as carbon source, and characterized by X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and UV–vis diffuse reflectance spectra (DRS). The photocatalytic results showed that (BiO)₂CO₃ display much higher photocatalytic activity than BiOCl and TiO₂ for photocatalystic reduction of CO₂ under UV-visible light. The photocatalytic mechanism study revealled that (BiO)₂CO₃ display better separation efficiency of photoinduced charge carriers due to the large interlayer spacing (1.3675 nm).     

High pulse repetition frequency RF excited waveguide CO2 laser with mechanical Q-switching

In this paper, a mechanical Q-switching is used in radio frequency (RF) excited waveguide CO₂ laser to obtain high pulse repetition frequency (PRF) laser. The Q-switching system includes two confocal ZnSe lenses and a high speed mechanical chopper, which is inserted into the cavity. The peak power is up to 730 W and the pulse width 200 ns at the highest PRF 20 kHz. The laser also has the advantages of compact, small-volume, and low-cost.